[compiler-rt] [compiler-rt][builtins] Fix detection of warnings flags (PR #125602)

[Sam James via llvm-commits]

https://github.com/thesamesam approved this pull request.

LGTM, but maybe let's do the other one case too:
```
compiler-rt/cmake/crt-config-ix.cmake:9:builtin_check_c_compiler_flag(-Wno-pedantic         COMPILER_RT_HAS_WNO_PEDANTIC)
```

https://github.com/llvm/llvm-project/pull/125602